This invention relates to improvements in data transmission, in particular data transmission along electric fences.
Electric fences are used for a number of purposes, the most well known applications are for managing livestock or for building or property security purposes. The common form of an electric fence is where one or more conductors are held in place by fence posts, at a suitable height above the ground. The conductors are electrically insulated from the ground by suitable application of insulating materials. The conductors are connected to an apparatus commonly known as an electric fence energiser, which periodically applies an electric pulse of high voltage (usually in the range of 1 kV to 10 kV) and of short duration (usually less than 1 ms) to the conductors. When touching the electric fence, the electric pulse is experienced as unpleasant or painful, forming an effective deterrent against crossing or applying pressure to the electric fence.
In many situations where electric fences are employed it is of advantage that the energiser can be remotely turned on and off. For example, a user may wish to make adjustments to an electric fence (e.g. repairs or change connections) at certain points of the fence, located at a considerable distance from the energiser. If this user had a means of remotely controlling the energiser then he or she would be able to do so with the energiser turned off and thereby avoid any risk of receiving an electric shock. When the adjustments have been made the user can turn the energiser back on immediately and check the result of the adjustments made.
It is known to use the conductor(s) of an electric fence as the medium for carrying the signals of the remote control system. It is also known to use this transmission of data for purposes other than remote control e.g. transmitting information about the water level in a tank located somewhere in the vicinity of the electric fence, information about the amplitude of the electric fence pulses at various locations along the electric fence line, etc.
For various reasons, known remote control systems relying on transmission of data along an electric fence conductor have suffered from drawbacks. For example, it has been suggested to use a low voltage DC signal to transmit along an electric fence, however, this method often performs very poorly on larger installations and on installations which receive limited servicing. This poor performance is often due to excessive attenuation of the low voltage DC signal by poor or defective wire joints along the electric fence.
It has been proposed to transmit data via a set of high voltage DC pulses along a fence line. The high voltage DC pulses are transmitted in a pattern that is “known” by the receivers' circuits. Thus the pulses are readily distinguished from natural electrical noise, man made electrical noise and from the high voltage pulses generated by the electric fence energiser. This system seeks to overcome the problem of attenuation of the signal by poor or defective wire joints by applying a sufficiently high voltage on the fence wire to form an arc across the gap.
Because it is desirable that the transmitter can be made a battery powered portable device and thus be hand held by the user, there are practical limitations that restrict the size and weight of the transmitting device. This in turn places practical limitations on the energy content of the high voltage pulses.
It has been suggested to use an AC signal to implement an electric fence remote control system. The system toggles a switch that enables, or disables the energiser once the presence of the AC signal has been detected on the conductors of the electric fence. The suggested arrangement is limited solely to controlling the energiser and does not include the option of transferring information for other useful functions.
A further proposed system uses a carrier frequency that is phase modulated with the information, along with a certain amount of extra information to enhance the reliability of the transmission method. The transmission method transmits the information as a group of digital bits or “data block”.
Phase modulation provides good performance when compared to many other modulation systems especially under circumstances where the signal may encounter distortion, attenuation and/or electrical noise. However, if a fence wire develops a defective wire joint where the electrical connection is actually broken by the two wires separating for a small distance eg. 0.1 mm-1 mm, the phase modulated signals suffer the same attenuation as the signals of the other methods previously described and in many cases the transmitted information cannot be recovered at the receiver.
A system for transmission of data along electric fences which performs poorly where small gaps and/or poor electrical wire joints are present is undesirable. Defective wire joints are very common in electric fences through the fence wires being continually exposed to climatic conditions and usually being under mechanical strain. Erosion and arcing are also common occurrences. Furthermore, electrical cut-out switches are often employed to enable the user to turn parts of the electric fence on or off. Many cut-out switches are simple mechanical devices and over time can develop poor electrical contacts or gaps caused by poor mechanical alignment, corrosion and/or dirt. However, the electric fence will usually continue to be effective since the voltage developed by the electric fence energiser is often high enough to form an arc across small gaps.